Prune Substitute

Hello all!
So nice to see you again. I came across older recipes that came out
before prune substitutes were available on the market (i.e. Wonderslim,
Sunsweet Lighter Bake). These older recipes call for making your own
using water and prunes. I am too lazy to make my own and would like to
substitute the prune puree. How do you determine how much prune puree
to use in place of the water and prunes?
Thanks.
Meryl
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